1. Field of the Invention
The present general inventive concept relates to a bi-component type developing unit and an image forming apparatus employing the same, and more particularly, to a developing unit which can prevent developer scattering and image loss and an image forming apparatus employing the same.
2. Description of the Related Art
In general, a bi-component developing unit uses a developer made of a toner for forming an image and a carrier for carrying the toner by a magnetic force, so as to develop an image. This developing unit includes a developing member having a magnetic cylinder and a sleeve installed to rotate on the cylinder, so as to separate the toner from the developer to form a toner image on an image support body. The magnetic cylinder has a catch pole for adsorbing the developer onto the sleeve, a main pole facing the image support body and directly contributing to development, a feeding pole for feeding the carrier remaining on the sleeve after the development, and a separating pole for separating the fed carrier from the sleeve.
FIG. 1 is a plan view illustrating a conventional developing unit, and FIG. 2 illustrates an axial magnetic force distribution of a magnetic cylinder with respect to a main pole, a catch pole and a feeding pole of the magnetic cylinder.
Referring to FIGS. 1 and 2, a magnetization width W1 of each of the plurality of magnetic poles constituting a magnetic cylinder 1a is the same as a width W2 of a developer layer formed on a sleeve 1b. A pair of sealing members 3 is provided between opposite end parts of the developer layer and opposite end parts of the sleeve 1b to prevent a developer D from leakage at opposite end parts of a developing member 1.
According to the above developing unit, a main pole N1 is configured such that magnetic forces A and B at opposite end parts of the main pole N1 are larger than a magnetic force C at a central part thereof due to an edge effect of a magnetic field generated at the opposite end parts of the main pole N1, as shown in FIG. 2. Thus, the thickness of a developer layer becomes high at the opposite end parts, and thus, developer distribution at a developing nip position of the image support body or at the main pole N1 extends toward the opposite end parts (refer to D1 in FIG. 1). Thus, the developer may leak at the opposite end parts of the main pole N1.
On the other hand, in a catch pole N2 and a feeding pole N3, magnetic forces at opposite end parts are smaller than those at central parts in a direction parallel to a longitudinal axis of the magnetic cylinder. Accordingly, the developer distribution is out of balance, and thus, the amount of the developer picked up at one end part of the catch pole N2 becomes deficient. Thus, as illustrated in FIG. 3, an image loss 11 may occur at one end part 10a of a developed image 10.